Miami Reef
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There’s also meq/L for alkalinity lolThere are two - SG for salinity and ppt for Alkalinity.
EDIT: ppm* not ppt
Reef Chemistry Question of the Day #282: Temperature Effects on Parameters
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DanyL
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True and it would be correct as well, but it is not commonly used (AFAIK) by reefers as Randy's hint suggested.
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For nitrate, I guess it depends upon what is meant by "PPM", since that unitless dimension is inherently ambiguous. In most cases, for saltwater analyses, I interpret "PPM" to mean "mg/L" (w/v), but it could be interpreted as "mg/kg" (w/w). In the former case, the value will change as the volume of the sample decreases, thus increasing the density of the sample. The actual value will go up, in terms of mg/L.
For salinity, I assume the question is asking about the actual parameter, and not its measured value, so since salinity is a measure of the salt content of the water, the actual salt content per kilogram will not change, so even though the measurement of salinity might be affected by the temperature change (whether the measurement method is based on conductivity, density, or refractive index), I'm going to identify this one as the one that does not really change with temperature.
For pH, the change in temperature will most certainly change the pKa of the various weak acids that make up the alkalinity, notably the carbonate/bicarbonate/carbonic acid system as well as the borate/boric acid system (not to mention the change in pKa of water itself), so pH will certainly change. It will go down.
For alkalinity, dKH is a conversion of meq/L, which is based on volume similar to the "mg/L" interpretation of PPM in the nitrate case, so I'm going to say that alkalinity will also change. The actual value will go up, in terms of meq/L.
For salinity, I assume the question is asking about the actual parameter, and not its measured value, so since salinity is a measure of the salt content of the water, the actual salt content per kilogram will not change, so even though the measurement of salinity might be affected by the temperature change (whether the measurement method is based on conductivity, density, or refractive index), I'm going to identify this one as the one that does not really change with temperature.
For pH, the change in temperature will most certainly change the pKa of the various weak acids that make up the alkalinity, notably the carbonate/bicarbonate/carbonic acid system as well as the borate/boric acid system (not to mention the change in pKa of water itself), so pH will certainly change. It will go down.
For alkalinity, dKH is a conversion of meq/L, which is based on volume similar to the "mg/L" interpretation of PPM in the nitrate case, so I'm going to say that alkalinity will also change. The actual value will go up, in terms of meq/L.
DanyL
Valuable Member
There’s another as well.
Hmm SC for salinity?
Seems to fit the partly volume based measurement (volume + conductivity) and AFAIK conductivity will be affected by temperature.
Miami Reef
Clam Fanatic
He said one of these has another unit of measurement that will have 2 answers depending on the measurement.
The real salinity (sg) will slightly change with temperature, but it won’t change with ppt. That’s my guess.
Miami Reef
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dennis romano
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I'm going for salinity
AydenLincoln
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Salinity!
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And the answer is...
Suppose your coral reef aquarium is chugging along nicely at 80 degrees F.
You withdraw a sample to run some tests, and the temperature drops to 72 degree F before you are ready.
Which of the following parameters will NOT have changed as the temperature changes from 80 to 72 degrees F (only considering the temp change, not anything else that may be happening, such as gas exchange with the air). Note that any change meets the question criteria. For example, an alk change from 7.4 dKH to 7.401 will count, even if it might still be written as 7.4 dKH.
1. Nitrate at 8 ppm
2. The salinity at 35 ppt
3. pH at 8.1
4. Alkalinity at 7.4 dKH
Several of you had these exactly right, but Let's work through the answers.
1 Nitrate at 8 ppm
ppm literally means parts per million. It can mean somewhat different things depending on the situation.
For a gas like CO2, for example, 400 ppm can mean 400 of the molecules in the gas are CO2 for every million total molecules in a given volume. The reason why it is number of molecule based here likely stems from gases usually being measured by pressure, and pressure it generally pretty will described by the total number of molecules present, not by their mass or some other attribute.
For a chemical dissolved in water, ppm nearly always is based on mass. There's no good reason why that needs to be, as opposed to by number of individual chemicals present, but it is the standard. So for nitrate at 8 ppm in seawater, it means there are 8 parts of nitrate by mass per million parts of seawater by mass. Or 8 mg per kg.
With a clarified definition of 8 ppm nitrate meaning 8 parts of nitrate in one million parts of seawater by mass, lets say you take a one kilogram sample of your tank water. No matter what temperature it is at, that same kilogram will always contain 8 mg of nitrate. Thus, nitrate cannot change with temperature WHEN MEASURED BY PPM.
As folks have indicated, and as was my intended hint, folks also sometimes report nitrate by mg/L. The relationship be3tween ppm and mg/L is quite close, but not exactly the same. 8 ppm means 8 mg/kg of seawater. But 1 kg of seawater occupies a bit less than 1 L of volume (around 0.977 L), so thet 8 ppm is close to 8.2 ppm. I and most folks ignore this difference in random reef chatting, but nevertheless, it is real.
Most importantly for this question, while 8 ppm is totally fixed with temperature, 8 mg/L is not, because the total mass of elements (including nitrate) in a liter of seawater varies with temperature as it expands and contracts.
Thus, nitrate is a case where the unit of measure impacts whether there is a temperature dependence.
2. Salinity at 35 ppt is a similar case to nitrate. 35 ppt literally means 35 parts per thousand, or 3.5% weight solids by mass. Thus, that sample is 3.5% solids and 96.5% water (ignoring weird chemical issues of what is a solid). With that definition in mind, the salinity of seawater measured in ppt is not ever going to change with temperature. IT will always have the same amount of solids and water in it.
There is only one other definition of salinity, and that is PSU. PSU is essentially supposed to be equivalent to ppt except it is measured by, and defined by, the conductivity. Such a measurement is supposed to be inherently temperature corrected, and I expect that a 35 PSU sample remains at 35 PSU regardless of temperature, but I have not looked into that deeply to say that with 100% certainty.
There are many ways to measure salinity, and many different devices that do it. These include hydrometers, refractometers, conductivity meters, etc. Some of these devices give ppt or PSU, and sometimes they give other units of measure, such as specific gravity. Specific gravity might be a useful surrogate measure for salinity, but it is not salinity by definition. Specific gravity changes a very, very small amount with temperature. The only reason it changes at all is because the density of seawater and the density of freshwater do not change exactly in lockstep as temp changes. However, many of the devices we use are very temperature dependent in their results, and may often need to be corrected for temp changes.
Final answer: salinity cannot change with temperature (at least if nothing is precipitating due to the temperature change).
More on the two wrong answers in a bit...
Suppose your coral reef aquarium is chugging along nicely at 80 degrees F.
You withdraw a sample to run some tests, and the temperature drops to 72 degree F before you are ready.
Which of the following parameters will NOT have changed as the temperature changes from 80 to 72 degrees F (only considering the temp change, not anything else that may be happening, such as gas exchange with the air). Note that any change meets the question criteria. For example, an alk change from 7.4 dKH to 7.401 will count, even if it might still be written as 7.4 dKH.
1. Nitrate at 8 ppm
2. The salinity at 35 ppt
3. pH at 8.1
4. Alkalinity at 7.4 dKH
Several of you had these exactly right, but Let's work through the answers.
1 Nitrate at 8 ppm
ppm literally means parts per million. It can mean somewhat different things depending on the situation.
For a gas like CO2, for example, 400 ppm can mean 400 of the molecules in the gas are CO2 for every million total molecules in a given volume. The reason why it is number of molecule based here likely stems from gases usually being measured by pressure, and pressure it generally pretty will described by the total number of molecules present, not by their mass or some other attribute.
For a chemical dissolved in water, ppm nearly always is based on mass. There's no good reason why that needs to be, as opposed to by number of individual chemicals present, but it is the standard. So for nitrate at 8 ppm in seawater, it means there are 8 parts of nitrate by mass per million parts of seawater by mass. Or 8 mg per kg.
With a clarified definition of 8 ppm nitrate meaning 8 parts of nitrate in one million parts of seawater by mass, lets say you take a one kilogram sample of your tank water. No matter what temperature it is at, that same kilogram will always contain 8 mg of nitrate. Thus, nitrate cannot change with temperature WHEN MEASURED BY PPM.
As folks have indicated, and as was my intended hint, folks also sometimes report nitrate by mg/L. The relationship be3tween ppm and mg/L is quite close, but not exactly the same. 8 ppm means 8 mg/kg of seawater. But 1 kg of seawater occupies a bit less than 1 L of volume (around 0.977 L), so thet 8 ppm is close to 8.2 ppm. I and most folks ignore this difference in random reef chatting, but nevertheless, it is real.
Most importantly for this question, while 8 ppm is totally fixed with temperature, 8 mg/L is not, because the total mass of elements (including nitrate) in a liter of seawater varies with temperature as it expands and contracts.
Thus, nitrate is a case where the unit of measure impacts whether there is a temperature dependence.
2. Salinity at 35 ppt is a similar case to nitrate. 35 ppt literally means 35 parts per thousand, or 3.5% weight solids by mass. Thus, that sample is 3.5% solids and 96.5% water (ignoring weird chemical issues of what is a solid). With that definition in mind, the salinity of seawater measured in ppt is not ever going to change with temperature. IT will always have the same amount of solids and water in it.
There is only one other definition of salinity, and that is PSU. PSU is essentially supposed to be equivalent to ppt except it is measured by, and defined by, the conductivity. Such a measurement is supposed to be inherently temperature corrected, and I expect that a 35 PSU sample remains at 35 PSU regardless of temperature, but I have not looked into that deeply to say that with 100% certainty.
There are many ways to measure salinity, and many different devices that do it. These include hydrometers, refractometers, conductivity meters, etc. Some of these devices give ppt or PSU, and sometimes they give other units of measure, such as specific gravity. Specific gravity might be a useful surrogate measure for salinity, but it is not salinity by definition. Specific gravity changes a very, very small amount with temperature. The only reason it changes at all is because the density of seawater and the density of freshwater do not change exactly in lockstep as temp changes. However, many of the devices we use are very temperature dependent in their results, and may often need to be corrected for temp changes.
Final answer: salinity cannot change with temperature (at least if nothing is precipitating due to the temperature change).
More on the two wrong answers in a bit...
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Continuing the discussion.
3. pH at 8.1
pH does change with temperature. All aqueous fluids do, and potentially to different extents. This effect is in addition to the temperature compensation that pH meters can do and often allow users to dial in. That correction is a correction of the probe response itself to the actual pH. But the actual pH also changes.
One main reason is that all acids and bases in the water will change strength to some extent as temp changes. Even water. In totally pure water, it breaks apart into H+ and OH-,
H2O <---> H+ + OH-
and in totally pure water at about 25 deg C, the concentration of H+ (and OH-) is about 10-7 moles/L. That is how the pH (the negative logarithm of the H+ concentration) of totally pure water comes out as pH 7.
As the temperature is raised, the concentration of H+ and OH- increases (the equation above shifts to the right, probably driven by entropy, being higher on the right with 2 ions vs one molecule on the left, since higher temps favor the higher entropy side of chemical equilibria, but I've not verified that expectation of mine in this particular circumstance). Thus, the pH declines (H+ rises) as temps rise even though the water is not any more acidic (having equal H+ and OH-).
In seawater, there are other acids and bases involved, complicating matters further. Bicarbonate, carbonate, carbonic acid, borate and boric acid, etc. All of these contribute to small changes in pH with temperature of seawater, even with nothing entering or leaving the water.
The exact, summed together and measured changes can be found in the scientific literature if folks are interested.
www.sciencedirect.com
3. pH at 8.1
pH does change with temperature. All aqueous fluids do, and potentially to different extents. This effect is in addition to the temperature compensation that pH meters can do and often allow users to dial in. That correction is a correction of the probe response itself to the actual pH. But the actual pH also changes.
One main reason is that all acids and bases in the water will change strength to some extent as temp changes. Even water. In totally pure water, it breaks apart into H+ and OH-,
H2O <---> H+ + OH-
and in totally pure water at about 25 deg C, the concentration of H+ (and OH-) is about 10-7 moles/L. That is how the pH (the negative logarithm of the H+ concentration) of totally pure water comes out as pH 7.
As the temperature is raised, the concentration of H+ and OH- increases (the equation above shifts to the right, probably driven by entropy, being higher on the right with 2 ions vs one molecule on the left, since higher temps favor the higher entropy side of chemical equilibria, but I've not verified that expectation of mine in this particular circumstance). Thus, the pH declines (H+ rises) as temps rise even though the water is not any more acidic (having equal H+ and OH-).
In seawater, there are other acids and bases involved, complicating matters further. Bicarbonate, carbonate, carbonic acid, borate and boric acid, etc. All of these contribute to small changes in pH with temperature of seawater, even with nothing entering or leaving the water.
The exact, summed together and measured changes can be found in the scientific literature if folks are interested.
The temperature dependence of pH in surface seawater
The temperature dependence of pH in surface seawater was investigated as a function of salinity and CO2 composition using the most recent values for t…
Last edited:
This is exactly what I was typing up but you're much faster! All kidding aside this is some interesting tid bits.
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Final answer discussion...
4. Alkalinity at 7.4 dKH
Alkalinity is a fairly complicated entity with a reasonably complicated definition that I won't get into here. I don't think we need to get into whether the carbonic acid pH endpoint of a pH titration changes with temperature in a closed system as it's a pretty intense discussion and I'm not certain of the answer for every acid/base ion in seawater.
But at its heart, it is a measure of a chemical attribute per unit volume. It may not appear so from dKH, but an equivalent and slight different unit is milliequivalents per liter (being like cm and inches for measuring length).
From the meq/L unit, it is clear there is a volume element, and that element changes with temperature as the water expands and contracts. Thus, alkalinity is expected to change with temperature to the extent that the water is expanding and contracting, and thus altering the number of alkalintiy contributing ions present in 1 L of the fluid.
Final answer: alkalinity is expected to change with temperature
4. Alkalinity at 7.4 dKH
Alkalinity is a fairly complicated entity with a reasonably complicated definition that I won't get into here. I don't think we need to get into whether the carbonic acid pH endpoint of a pH titration changes with temperature in a closed system as it's a pretty intense discussion and I'm not certain of the answer for every acid/base ion in seawater.
But at its heart, it is a measure of a chemical attribute per unit volume. It may not appear so from dKH, but an equivalent and slight different unit is milliequivalents per liter (being like cm and inches for measuring length).
From the meq/L unit, it is clear there is a volume element, and that element changes with temperature as the water expands and contracts. Thus, alkalinity is expected to change with temperature to the extent that the water is expanding and contracting, and thus altering the number of alkalintiy contributing ions present in 1 L of the fluid.
Final answer: alkalinity is expected to change with temperature
DanyL
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Thank you for the detailed explanation Randy, I definitely learned a thing or two
I guess I leaned a bit too much towards the measurement methods at the end, but I was correct about the part-per-* units
I've read more about PSU (PSS-78) but one thing I couldn't understand is that wikipedia for some reason says it shouldn't be used as unit, which kinda left me baffled. Do you know what they mean by any chance? My first guess was about the fixed temperature but it doesn't add up. If anything, that's what makes it a more correct unit to describe measured salinity compared to most other units where you also need to specify the temp used to measure.
My second guess was because the name of the unit is already in use for a different kind of measurement, but they would simply name it differently if it was a problem in the first place.
I guess I leaned a bit too much towards the measurement methods at the end, but I was correct about the part-per-* units
I've read more about PSU (PSS-78) but one thing I couldn't understand is that wikipedia for some reason says it shouldn't be used as unit, which kinda left me baffled. Do you know what they mean by any chance? My first guess was about the fixed temperature but it doesn't add up. If anything, that's what makes it a more correct unit to describe measured salinity compared to most other units where you also need to specify the temp used to measure.
My second guess was because the name of the unit is already in use for a different kind of measurement, but they would simply name it differently if it was a problem in the first place.
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Thank you for the detailed explanation Randy, I definitely learned a thing or two
I guess I leaned a bit too much towards the measurement methods at the end, but I was correct about the part-per-* units
I've read more about PSU (PSS-78) but one thing I couldn't understand is that wikipedia for some reason says it shouldn't be used as unit, which kinda left me baffled. Do you know what they mean by any chance? My first guess was about the fixed temperature but it doesn't add up. If anything, that's what makes it a more correct unit to describe measured salinity compared to most other units where you also need to specify the temp used to measure.
My second guess was because the name of the unit is already in use for a different kind of measurement, but they would simply name it differently if it was a problem in the first place.
I think it just means you should say “the salinity is 35” and not “the salinity is 35 PSU”.
DanyL
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Oh right, rereading it again I think you are correct.
Their explanation was a bit vague, but could also be just a language barrier on my side.
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